This simply means that something was sent to the browser prior to the headers being sent as part of your request. The two times you'll see this the most are:
1 - You've had another error that is displaying something on your page. Fix the error and it will go away.
2 - You have a space or a blank line or other text before the opening `<?php` of a file. Go through your controller, libraries, models, etc, that are being used on that page and make sure there isn't anything before that tag at the top of the file.
